Electric vehicles in which power of batteries thereof is boosted by converter circuits and supplied to motors have been known. These types of electric vehicles each have an LC circuit configured by a smoothing capacitor that smoothens a voltage increased by the converter circuit and a reactor of the converter circuit. A resonance of the LC circuit (LC resonance) causes a current flowing therethrough to oscillate with a large amplitude and at a high frequency, overheating parts of the electric vehicle and deteriorating the battery. For this reason, the occurrence of an LC resonance needs to be prevented as much as possible in order for the electric vehicle to operate.
In many electric vehicles, sine-wave PWM control system and square-wave control system are used in combination as a motor control system. The sine-wave PWM control system is used when a motor of an electric vehicle operates in a low-speed/low-torque region. The square-wave control system is used when the motor operates in a high-speed/high-torque region. The sine-wave PWM control system does not cause the LC resonance because an inverter circuit is switched on a cycle based on a carrier frequency higher than a resonance frequency of the LC circuit in this system. The square-wave control system, on the other hand, causes the LC resonance, depending on a rotational speed of the motor, because the inverter circuit is switched on an interruption cycle based on the rotational speed of the motor.
Patent document 1 discloses a technology for avoiding an LC resonance when the square-wave control system described above is used. According to patent document 1, when an operating point of a motor is included in a predetermined resonance region, a voltage of an inverter circuit is increased to be higher than a voltage of a battery by a converter circuit, and the inverter circuit is controlled using sine-wave PWM control system. The resonance region here means an operating region of the motor where an LC circuit might resonate when the motor operates at its torque and rotational speed. The technology disclosed in patent document 1 boosts the voltage at the converter circuit to expand a region to which the sine-wave PWM control system can be applied. Therefore, the square-wave control system is not used in the resonance region to which the sine-wave PWM control system is applied. The technology disclosed in patent document 1 is capable of preventing the occurrence of the LC resonance because the square-wave control system is not used when the resonance region includes the operating point of the motor.